This invention relates to the production of television images for use with chroma-key, a known technique in which foreground subjects are located in front of a backing of a particular color the presence of which is detected in a television image and caused to substitute a different television image of a background scene to produce the effect that the subjects are located in front of this background. Such a technique is described in U.S. Pat. No. 2,974,190.
A major drawback to the present use of chroma-key is that if the camera is moved to follow motion of the subjects the image of the background remains stationary, producing an artificial effect of the subjects floating in front of the background image.
This invention aims to provide a background image, which may include moving objects, which will follow motion of the foreground camera so that it appears to be viewed by this camera.
Prior efforts to produce this result have used a photographic background mounted on a movable easel which is servo-controlled to follow motions of the camera. Such a system is described in U.S. Pat. No. 4,202,008. This is obviously limited to still backgrounds. A version of this system using a servo-controlled motion-picture projector as background image source is also described in this patent. This approach is limited by the area of the scene included in the film frame, and by jitter of the motion-picture image. Other efforts to produce the desired effect have employed an electronic frame store capable of repositioning a television image. A system of this type is described in U.S. Pat. No. 4,200,890. Frame stores are expensive and introduce delay which causes motion of the image to lag behind motion of the foreground camera. Also the field of view is limited to that contained in the television image, consequently moving this image leaves a blank area at one or other side. This restricts use to cases where the repositioned image does not fill the full width and height of the final combined image, such as a chroma-key insert into a scene, as referred to in the referenced U.S Patent.